1. Field of the Invention
The invention relates to a signal source device, and more particularly to a signal source device applied in a transceiver.
2. Description of the Related Art
The global system for mobile communications (GSM) is the most widely used cellular phone system. Phase noise is an important specification for a 900-MHz GSM signal source device of a transmitter (TX) in a GSM transceiver. Particularly, because the phase noise of the signal source device at 20 MHz offset will interfere with the receiving band of a receiver (RX), the phase noise of the signal source device is strictly restricted. According to GSM specifications, the phase noise of the local oscillator (LO) at 20 MHz is restrained below −162 dBc/Hz. GSM transceiver designs have been developed toward single CMOS chip solutions. However, for the latest CMOS technology, there are some problems which include: low quality on-chip inductors, lossy Si substrate, and increased 1/f noise accompanying with the channel scale-down. The problems seriously affect the phase noise of the signal source device, such that there are no assurances, that the phase noise of the signal source device at 20 MHz offset will always be below −162 dBc/Hz.
Thus, it is desired to provide a signal source device which provides an output signal with low phase noise. Especially, when the provided signal source device is applied in a GSM transceiver, and the output signal conforms to GSM specifications.